It seems to work so far though I'm only level 21 and I'm too vulnerable to almost dying during the faceroll when going up against level 24-25 opponents, let alone getting stunned out of the combo or misplacing the fire field (way too common grrr I suck at this game) and having no backup plan other than kiting in circles with lightning.

So to survive I would get signet of earth (instead of CF, meaning only 18 stacks instead of 21 qq) and 20 points into earth including two defensive traits.

Would this be enough? Should I put points into water instead which is mathematically better to increase survival? Is signet of earth worth it just for the passive? Is it even viable at higher levels?

Seems viable but I reckon there are better alternatives. One thing though, don't overrate the power of Might stacks. With all those stacks we're talking about around 10% extra damage. And that is only temporary as well. You can't keep that amount of stacks going all the time because the duration does not get renewed with new stacks.

Using only the fire and air attunements you're missing out on some very powerful skills in water and earth. I suggest you don't limit the build like that and take whatever fits in your playstyle considering the skills offered in every attunement. Also a cantrip > 3 might stacks seems a bit like overkill since you can already get a good amount from your fire field combo. Better off spending the trait elsewhere I think. More so because you only carry one cantrip.

One last thing, if this build is for PVP I would also consider figuring out your stat allocation (or: your PVP gear set) and the stuff you put on your armor and weapons. There are runes with +15% boon duration and 20% might duration, for example. And they stack too.